Huntington's disease is a neurodegenerative disorder caused by an expanding CAG repeat coding for polyglutamine in the huntingtin gene. In the previous grant period we established and studied a transgenic mouse model of HD using an N-terminal fragment of huntingtin. We have now developed a new inducible transgenic model of HD expressing full-length huntingtin with an N-terminal myc tag to facilitate biochemical and cell biological studies. Preliminary data indicate that the mice have a robust progressive behavioral phenotype and characteristic pathology, as well as the presence of an N-terminal fragment of huntingtin. We believe proteolytic cleavage may be a critical event in HD pathogenesis. We study this HD model in parallel with a mouse model of the closely related polyglutamine disorder DRPLA, in which proteolytic processing appears to be critical for pathogenesis. In specific aim 1 we will breed sufficient numbers of these HD inducible transgenic mice to characterize the time course of the behavioral phenotype and to provide tissue for pathological and biochemical studies. In specific aim 2 we will define the pathology of the mice, including the regional distribution of intranuclear inclusions and other aggregates of the huntingtin protein. In specific aim 3 we will study the time course and regional and cellular distribution of the huntingtin fragment. We will determine its length and use methods of protein purification and mass spectrometry to determine the cleavage site. We will develop a cell model with stable inducible expression of the same mutant huntingtin construct for comparative in vitro studies. In specific aim 4 we will generate new transgenic mice with alterations in the huntingtin protein sequence to eliminate proteolytic cleavage. We predict that these mice will have a substantially decreased HD phenotype. Taken together these studies will establish cleavage as a key event in HD pathogenesis, and as a novel and accessible target for therapeutics. The new mouse model will be useful for studying pathogenic features of HD, and for testing experimental therapeutic interventions.